The present invention is related to power beam receivers, and more specifically to diffusing concentrator for a power beam receiver.
Currently problems arise in transmitting power via beams of light because beams of light from artificial sources are not like sunlight. Sunlight has nearly uniform intensity over areas smaller than the Earth. Standard practice for arrays of photovoltaic cells (often called “solar cells”) is to assume they will be used with uniform illumination. Hence, it is assumed that each cell produces the same power as its neighbors: cells are wired in series where each cell passes the same current and imparts the same voltage rise as its neighbors. With an artificial beam, this is not the case. Intensity of a beam varies across the beam width, often as a nominal Gaussian or Bessel function of radius from the beam center. The width of the beam may vary with distance from the source. In addition to the nominal variation, the beam pattern may have fine-grained variations induced by atmospheric turbulence. As a result, the beam that reaches a photovoltaic (PV) array may be highly non-uniform.
One cell, B, may receive substantially less light than another cell, A. In that case, cell B would produce substantially less current than A. (To a first approximation, a photovoltaic cell can be treated as a current generator whose output current is proportional to its illumination.) If A and B are wired in series, cell B blocks much of the current that cell A would normally produce. Therefore, even though cell A receives substantially more light than cell B, cell A's power output is reduced to barely more than B's. This reduces the overall efficiency of converting light to electricity. In an extreme case, one un-illuminated cell in a string can prevent all other cells in the string from producing power, i.e. efficiency is zero.
If one could predict exactly what beam pattern would occur, where it would be centered on the array, and what incidence angle it would have, then we could design an array where the size, efficiency, and orientation of each cell is selected so that all cells in a series string produce the same output current. In reality, however, the beam pattern may vary due to atmospheric irregularities and deformation of optical surfaces; its center location jitters due to imperfect tracking; and the angle of incidence varies as the receiving vehicle moves relative to the beam source. Furthermore, for a diffraction-limited beam, the beam width is roughly proportional to distance from the beam source, so the beam width will vary as the receiving vehicle moves closer to or farther from the beam source. If the beam width is wider than the PV array, then some of the beam's energy misses the array and is lost; if the beam width is narrower than the PV array, then some of the PV cells are not illuminated, causing problems as described above. Overall, then, the uneven, unpredictable illumination of a PV array by a power beam causes lost efficiency in several ways.
There are some current solutions for this particular problem, for example, wide, over-powered laser, local MPP controller, series-parallel trellis, and optical concentrator. However, each of these current solutions is problematic.
The wide, over-powered laser solution uses a greatly over-powered laser to illuminate the vehicle with a wide beam that is locally uniform and smooth over the width of the PV array. However, high-power lasers are costly: $10/watt is near the minimum cost, not including the cost of a highly regulated electrical power source, a reliable cooling system, beam-forming optics, and beam-steering optics and electronics. Even the energy supply for a laser can be costly, especially if the laser is used on a vehicle or in a remote location. Using a beam that is wide enough to project a near-uniform sub-pattern on the receiver wastes most of the laser's power. In addition, the user would have to pay for the whole high-power system. Besides the cost penalty, this solution does not fully solve the problem of non-uniform illumination. Atmospheric turbulence causes fleeting, small patches of brighter or dimmer illumination on the receiver, wasting energy due to mismatched output from PV cells in series.
The local MPP controller solution gives every cell a maximum power point (MPP) controller and DC-DC converter. An MPP controller continuously adjusts the cell's output load to maximize the cell's output power (current x voltage). The DC-DC converter transforms the voltage from each cell (which may vary as the cell's illumination varies) to match the common voltage at which all cells' current is collected in parallel. However, MPP controllers and DC-DC converters are not very efficient, especially at the low power levels produced by a single cell. Therefore, this solution wastes a substantial fraction of the beam power as heat. In addition, adding two electronic devices to each cell or small group of cells dramatically increases the weight and cost of a PV array.
The series-parallel trellis solution uses a complex series-parallel wiring scheme to reduce the effect of series-connection bottlenecks from under-illuminated cells. However, this solution requires many additional wires which add substantial weight to the PV array, and it is costly to design with current methods.
The optical concentrator solution uses an oversized optical concentrator to collect light from a wide beam and concentrate it on a modest-sized PV array. It may use reflecting or refracting optics. This solution mitigates the loss of energy from a beam that is substantially wider than the PV array. However, this solution (a) does not solve the problem of non-uniformity within the beam and (b) creates non-uniformity for beams smaller than the concentrator. For example, regarding (a), a wide, non-uniform beam is fully captured by the concentrator but its projection onto the PV array has the same pattern of non-uniformity as the original beam, just smaller. Also, regarding (b), a beam that is narrower than the concentrator is focused onto a subset of the PV array. Even if the beam were perfectly uniform, this focusing effect would leave part of the array dark.
In addition, there is a related problem of providing diffuse lighting in a small area for detailed manual work, such as dentistry or surgery. The current solution is a diffusing concentrator near the light source. A web search on the term “surgical lamp” will find many examples. Though the surgical lamp is an effective solution to the medical problem, it produces a beam that is too wide for high-intensity illumination at long range, and is, therefore, not an effective solution to the beamed power problem.